A diesel engine is a type of internal combustion engine that uses compression ignition to burn fuel. It was invented by German engineer Rudolf Diesel in the late 19th century and is widely used in transportation, industry, and agriculture. In a diesel engine, fuel is injected into the combustion chamber where it mixes with hot, compressed air. The mixture ignites spontaneously due to the high pressure and temperature, causing the piston to move and drive the engine. Diesel engines are known for their high fuel efficiency and longevity, making them popular in commercial vehicles such as trucks, buses, and trains.

Diesel engines are also used in stationary applications such as power generators and industrial machinery. However, diesel engines are known for their emissions of particulate matter and nitrogen oxides, which can contribute to air pollution and have negative health effects. In recent years, efforts have been made to improve the emissions performance of diesel engines through the use of cleaner fuels, improved engine designs, and after-treatment technologies.